Silicon-Controlled Rectifiers (SCRs) are a type of thyristor that can carry and control large currents utilizing a low voltage/current control signal. SCR's are used to control AC current for lighting or motor drive, for circuit protection as “crowbars” or as rectifiers in DC power supplies. They have been referred to as the workhorse for high power applications where high current or voltage is switched and controlled, especially in industrial applications. In industrial heating, for example, many furnaces utilize an SCR based controller to control the power delivered to the heating element or the electrodes.
SCRs are typically controlled with two methods; zero-cross, and phase angle. In the zero-cross control method, the SCR is triggered after the voltage has crossed zero and due to its construction, the SCR will be on for the subsequent half cycle. This technique is useful in applications with an emphasis on electromagnetic emissions. This technique is limited in resolution since the SCR will always be conducting for at least half cycle every time a gate trigger signal is available.
For higher resolution applications, the second method is more in use where the SCR is triggered at a specific firing angle after zero-cross. This technique gives the ability to trigger at any time in the cycle when the SCR is forward biased. This technique has the advantage of providing for the highest resolution. However, in practice it is limited by the resolution of the controller and the firing circuitry which includes a timer for the firing of pulse trains. Furthermore, new heating and power applications require higher power outputs and extremely broad control ranges with higher resolutions. Additionally most control systems are moving towards digital control systems, which adds a degree of complexity and additional sources of errors and inaccuracies.
A typical SCR based DC or AC power supply will have the SCRs placed either in the primary side of a transformer or secondary. A block diagram of a common prior art DC power supply is shown in FIG. 1.
In this Figure, a user interface 12, which can be remote where the end user enters or programs the desired power, voltage or current output then communicate it to the system controller 14 via either an analog or a digital link 16. The interface 12 can also be local to the power supply though an HMI, a keypad or a dial. The desired output (the setpoint) is communicated to the control system 14 which can be supervised by either a microcontroller, a Digital signal Processor (DSP), a Field Programmable Gate Array (FPGA), an analog controller or a Programmable Logic Controller (PLC).
The control system 14 outputs either an analog signal or a digital signal 18 that represents the firing angle. This signal 18 is transmitted to the firing circuitry 20 where the signal 18 is interpreted and a gate firing signal 22 is generated to allow current to flow through the SCRs 24 to the load 26 for the specified amount of time.
Typically the output of an Analog-to-digital converter (ADC) is converted into a signal equivalent to a firing angle from 0 to 180°. For example, a 10 bit ADC has a full range of 1024 counts which means a resolution [R] of
  R  =            180      1024        =                  0.175        °            /              count        .            To put this number in perspective, one can analyze the relationship between phase angle and voltage which is written as follows:
                              V          oRMS                =                              3                    ⁢                      V            s                    ⁢                                    (                                                1                  2                                +                                                                            3                      ⁢                                              3                                                                                    4                      ⁢                                                                                          ⁢                      π                                                        ⁢                  cos                  ⁢                                                                          ⁢                  2                  ⁢                                                                          ⁢                  α                                            )                                                          (        1        )            
Here VoRMS is the output voltage, Vs the source voltage and α is the firing angle. Since most heating applications prefer power control, a more important factor is the power resolution, where the relationship between firing angle and resolution is as follows:
                              P          o                =                                            3              ⁢                              V                s                2                                      R                    ⁢                      (                                          1                2                            +                                                                    3                    ⁢                                          3                                        ⁢                    θ                                                        4                    ⁢                                                                                  ⁢                    π                                                  ⁢                cos                ⁢                                                                  ⁢                2                ⁢                α                                      )                                              (        2        )            
In theory this equation shows a very high resolution and accuracy, however, in practice a 1% accuracy is very difficult to achieve and a 1° resolution is almost impossible. A 1° resolution is equivalent to 120 W in a 100 kW power system and 300 W in a 500 kW power system.
Accordingly, what is needed is a method and system to provide high resolution power control in SCR based power supplies.